This invention relates to an optical device for achieving single longitudinal mode laser emission.
At present, great interest is being directed to the production of single longitudinal mode (single wavelength) light output from a semiconductor laser. One single wavelength technique, known as distributed Bragg reflection (DBR), is achieved by periodic waveguide reflection or waveguide grating reflection as an alternative to Fresnel reflection from the facets of a semiconductor laser. Light in a narrow frequency band is reflected from the gratings back into the laser active layer thereby selecting a longitudinal mode within the reflection band for oscillation from the multiple longitudinal modes which would normally oscillate within the gain bandwidth of the laser. In another form of Bragg reflection, a grating may be supplied external to the laser rather than as an integrated portion of the semiconductor.
Another class of techniques for achieving single frequency oscillation is based on the use of coupled optical cavities. The resonant frequencies of the two (or more) cavities are such that only one oscillation frequency is allowed within the gain bandwidth of the laser. Recently, a promising version of this approach has been suggested where the two cavities are achieved by cleaving a standard two-mirror laser diode and introducing an internal reflection surface. The cavities are also electrically isolated so that the wavelength can be adjusted by changing the optical length of the cavities by varying the current to one or both cavities. (See, U.S. patent application of W. Tsang, Ser. No. 482,964, filed Apr. 8, 1983, and assigned to the present assignee.) While this approach appears to be an excellent solution to the problem of producing single-mode emission, it, like the DBR laser, requires modification of the simple two-mirror laser diode. Another approach is the use of a Fabry-Perot interferometer provided external to the laser structure so that light reflected by the interferometer onto the laser will closely match only one mode of the laser. One difficulty with this technique is the fact that the interferometer provides a fairly broad band, weakly-wavelength-dependent, reflection which may result in instabilities in the mode selection.
It is desirable, as an alternative, to provide a relatively simple, external, passive element which will reflect efficiently in only a narrow wavelength band so that when coupled to a facet of a simple laser diode, the laser will be constrained to emit at only a single frequency. Although narrow band reflective filters have been proposed for selection of lasing lines and modes in a CO.sub.2 laser, apparently no one has previously taught how single mode emission can be produced from a multimode semiconductor laser with a reflective filter. (See, for example, Leeb, "Tunable Metal Film Filters as Narrow Band IR Laser Reflectors," Applied Optics, Vol. 15, No. 3, pp. 681-689 (March 1976) and Leeb, "CO.sub.2 Laser Line Selection With Metal Film Reflector," AEU Archive Elek. Ueber, Vol. 32, pp. 186-190 (1978).)
It is therefore a primary object of the invention to provide an optical device comprising a semiconductor laser and filter combination which is capable of producing single-mode emission.